A λ/4 plate and a λ/2 plate have many uses in relation to anti-reflection films and liquid crystal displays, and hence have been widely and practically used. However, in spite of the name of λ/4 or λ/2, most λ/4 or λ/2 plates give λ/4 or λ/2 at particular wavelengths, respectively.
Japanese Patent Provisional Publication Nos. 5(1993)-27118 and 5(1993)-27119 disclose a phase-retarder in which a birefringencial film giving high retardation and another birefringencial film giving low retardation are laminated so that their optical axes may be perpendicularly crossed. If the retardation difference of those films is kept λ/4 in the whole visible wavelength region, the phase-retarder theoretically gives λ/4 in the whole visible wavelength region.
Japanese Patent Provisional Publication No. 10(1998)-68816 discloses a λ/4 plate giving λ/4 in a wide wavelength region. The disclosed λ/4 plate comprises laminated two films made of the same polymer, and one of the films gives λ/4 and the other gives λ/2 at the same wavelength.
Japanese Patent Provisional Publication No. 10(1998)-90521 also describes another wide-ranging λ/4 plate comprising laminated two polymer films.
As the polymer films used in the above λ/4 plates, stretched films of synthetic polymer such as polycarbonate are used.
Japanese Patent Provisional Publication No. 2000-137116 discloses still another λ/4 plate comprising a stretched cellulose ester film for giving λ/4 in a wide wavelength region.
International Patent WO00/65384 discloses yet another λ/4 plate comprising a cellulose ester film in which an aromatic compound (retardation-increasing agent) is incorporated so as to give λ/4 in a wide wavelength region.